Adaptive weighting of reference pictures in video decoding

ABSTRACT

A video decoder, encoder, and corresponding methods for processing video signal data for an image block and a particular reference picture index to predict the image block are disclosed that utilize adaptive weighting of reference pictures to enhance video compression, where a decoder includes a reference picture weighting factor unit for determining a weighting factor corresponding to the particular reference picture index; an encoder includes a reference picture weighting factor assignor for assigning a weighting factor corresponding to the particular reference picture index; and a method for decoding includes receiving a reference picture index with the data that corresponds to the image block, determining a weighting factor for each received reference picture index, retrieving a reference picture for each index, motion compensating the retrieved reference picture, and multiplying the motion compensated reference picture by the corresponding weighting factor to form a weighted motion compensated reference picture.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. Non-Provisionalpatent application Ser. No. 10/410,481, filed Apr. 9, 2003; which itselfclaims the benefit of of U.S. Provisional Patent Application Ser. No.60/395,843 filed Jul. 15, 2002; and also U.S. Provisional PatentApplication Ser. No. 60/395,874 filed Jul. 15, 2002, and all of whichare incorporated by reference herein in their respective entireties.

FIELD OF THE INVENTION

The present invention is directed towards video decoders, and inparticular, towards utilization of adaptive weighting of referencepictures in video decoders.

BACKGROUND OF THE INVENTION

Video data is generally processed and transferred in the form of bitstreams. Typical video compression coders and decoders (“CODECs”) gainmuch of their compression efficiency by forming a reference pictureprediction of a picture to be encoded, and encoding the differencebetween the current picture and the prediction. The more closely thatthe prediction is correlated with the current picture, the fewer bitsthat are needed to compress that picture, thereby increasing theefficiency of the process. Thus, it is desirable for the best possiblereference picture prediction to be formed.

In many video compression standards, including Moving Picture ExpertsGroup (“MPEG”)-1, MPEG-2 and MPEG-4, a motion compensated version of aprevious reference picture is used as a prediction for the currentpicture, and only the difference between the current picture and theprediction is coded. When a single picture prediction (“P” picture) isused, the reference picture is not scaled when the motion compensatedprediction is formed. When bi-directional picture predictions (“B”pictures) are used, intermediate predictions are formed from twodifferent pictures, and then the two intermediate predictions areaveraged together, using equal weighting factors of (½, ½) for each, toform a single averaged prediction. In these MPEG standards, the tworeference pictures are always one each from the forward direction andthe backward direction for B pictures.

SUMMARY OF THE INVENTION

These and other drawbacks and disadvantages of the prior art areaddressed by a system and method for adaptive weighting of referencepictures in video decoders.

A video decoder and corresponding methods are disclosed for decodingvideo data for a picture having a plurality of image blocks. The videodata includes data for an image block of the plurality of image blocksand includes a first reference picture index and a second referencepicture index. The video data is received and the received video dataincludes the data for the image block, the first reference picture indexfor the image block, and the second reference picture index for theimage block. The first reference picture index corresponds to a firstreference picture, and the second reference picture index corresponds toa second reference picture. The image block is predicted using the firstand second reference pictures, and using a first offset corresponding tothe first reference picture index, and using a second offsetcorresponding to the second reference picture index. The first referencepicture index determines both the first reference picture and the firstoffset, and the second reference picture index determines both thesecond reference picture and the second offset.

These and other aspects, features and advantages of the presentinvention will become apparent from the following description ofexemplary embodiments, which is to be read in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Adaptive weighting of reference pictures in video coders and decoders inaccordance with the principles of the present invention are shown in thefollowing exemplary figures, in which:

FIG. 1 shows a block diagram for a standard video decoder;

FIG. 2 shows a block diagram for a video decoder with adaptivebi-prediction;

FIG. 3 shows a block diagram for a video decoder with reference pictureweighting in accordance with the principles of the present invention;

FIG. 4 shows a block diagram for a standard video encoder;

FIG. 5 shows a block diagram for a video encoder with reference pictureweighting in accordance with the principles of the present invention;

FIG. 6 shows a flowchart for a decoding process in accordance with theprinciples of the present invention; and

FIG. 7 shows a flowchart for an encoding process in accordance with theprinciples of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention presents an apparatus and method for motion vectorestimation and adaptive reference picture weighting factor assignment.In some video sequences, in particular those with fading, the currentpicture or image block to be coded is more strongly correlated to areference picture scaled by a weighting factor than to the referencepicture itself. Video CODECs without weighting factors applied toreference pictures encode fading sequences very inefficiently. Whenweighting factors are used in encoding, a video encoder needs todetermine both weighting factors and motion vectors, but the best choicefor each of these depends on the other, with motion estimation typicallybeing the most computationally intensive part of a digital videocompression encoder.

In the proposed Joint Video Team (“JVT”) video compression standard,each P picture can use multiple reference pictures to form a picture'sprediction, but each individual motion block or 8×8 region of amacroblock uses only a single reference picture for prediction. Inaddition to coding and transmitting the motion vectors, a referencepicture index is transmitted for each motion block or 8×8 region,indicating which reference picture is used. A limited set of possiblereference pictures is stored at both the encoder and decoder, and thenumber of allowable reference pictures is transmitted.

In the JVT standard, for bi-predictive pictures (also called “B”pictures), two predictors are formed for each motion block or 8×8region, each of which can be from a separate reference picture, and thetwo predictors are averaged together to form a single averagedpredictor. For bi-predictively coded motion blocks, the referencepictures can both be from the forward direction, both be from thebackward direction, or one each from the forward and backwarddirections. Two lists are maintained of the available reference picturesthat may used for prediction. The two reference pictures are referred toas the list 0 and list 1 predictors. An index for each reference pictureis coded and transmitted, ref_idx_I0 and ref_idx_I1, for the list 0 andlist 1 reference pictures, respectively. Joint Video Team (“JVT”)bi-predictive or “B” pictures allows adaptive weighting between the twopredictions, i.e.,Pred=[(P0)(Pred0)]+[(P1)(Pred1)]+D,where P0 and P1 are weighting factors, Pred0 and Pred1 are the referencepicture predictions for list 0 and list 1 respectively, and D is anoffset.

Two methods have been proposed for indication of weighting factors. Inthe first, the weighting factors are determined by the directions thatare used for the reference pictures. In this method, if the ref_idx_I0index is less than or equal to ref_idx_I1, weighting factors of (½, ½)are used, otherwise (2, −1) factors are used.

In the second method offered, any number of weighting factors istransmitted for each slice. Then a weighting factor index is transmittedfor each motion block or 8×8 region of a macroblock that usesbi-directional prediction. The decoder uses the received weightingfactor index to choose the appropriate weighting factor, from thetransmitted set, to use when decoding the motion block or 8×8 region.For example, if three weighting factors were sent at the slice layer,they would correspond to weight factor indices 0, 1 and 2, respectively.

The following description merely illustrates the principles of theinvention. It will thus be appreciated that those skilled in the artwill be able to devise various arrangements that, although notexplicitly described or shown herein, embody the principles of theinvention and are included within its spirit and scope. Furthermore, allexamples and conditional language recited herein are principallyintended expressly to be only for pedagogical purposes to aid the readerin understanding the principles of the invention and the conceptscontributed by the inventor to furthering the art, and are to beconstrued as being without limitation to such specifically recitedexamples and conditions. Moreover, all statements herein recitingprinciples, aspects, and embodiments of the invention, as well asspecific examples thereof, are intended to encompass both structural andfunctional equivalents thereof. Additionally, it is intended that suchequivalents include both currently known equivalents as well asequivalents developed in the future, i.e., any elements developed thatperform the same function, regardless of structure.

Thus, for example, it will be appreciated by those skilled in the artthat the block diagrams herein represent conceptual views ofillustrative circuitry embodying the principles of the invention.Similarly, it will be appreciated that any flow charts, flow diagrams,state transition diagrams, pseudocode, and the like represent variousprocesses which may be substantially represented in computer readablemedia and so executed by a computer or processor, whether or not suchcomputer or processor is explicitly shown.

The functions of the various elements shown in the figures may beprovided through the use of dedicated hardware as well as hardwarecapable of executing software in association with appropriate software.When provided by a processor, the functions may be provided by a singlededicated processor, by a single shared processor, or by a plurality ofindividual processors, some of which may be shared. Moreover, explicituse of the term “processor” or “controller” should not be construed torefer exclusively to hardware capable of executing software, and mayimplicitly include, without limitation, digital signal processor (“DSP”)hardware, read-only memory (“ROM”) for storing software, random accessmemory (“RAM”), and non-volatile storage. Other hardware, conventionaland/or custom, may also be included. Similarly, any switches shown inthe figures are conceptual only. Their function may be carried outthrough the operation of program logic, through dedicated logic, throughthe interaction of program control and dedicated logic, or evenmanually, the particular technique being selectable by the implementeras more specifically understood from the context.

In the claims hereof any element expressed as a means for performing aspecified function is intended to encompass any way of performing thatfunction including, for example, a) a combination of circuit elementsthat performs that function or b) software in any form, including,therefore, firmware, microcode or the like, combined with appropriatecircuitry for executing that software to perform the function. Theinvention as defined by such claims resides in the fact that thefunctionalities provided by the various recited means are combined andbrought together in the manner which the claims call for. Applicant thusregards any means that can provide those functionalities as equivalentto those shown herein.

As shown in FIG. 1, a standard video decoder is indicated generally bythe reference numeral 100. The video decoder 100 includes a variablelength decoder (“VLD”) 110 connected in signal communication with aninverse quantizer 120. The inverse quantizer 120 is connected in signalcommunication with an inverse transformer 130. The inverse transformer130 is connected in signal communication with a first input terminal ofan adder or summing junction 140, where the output of the summingjunction 140 provides the output of the video decoder 100. The output ofthe summing junction 140 is connected in signal communication with areference picture store 150. The reference picture store 150 isconnected in signal communication with a motion compensator 160, whichis connected in signal communication with a second input terminal of thesumming junction 140.

Turning to FIG. 2, a video decoder with adaptive bi-prediction isindicated generally by the reference numeral 200. The video decoder 200includes a VLD 210 connected in signal communication with an inversequantizer 220. The inverse quantizer 220 is connected in signalcommunication with an inverse transformer 230. The inverse transformer230 is connected in signal communication with a first input terminal ofa summing junction 240, where the output of the summing junction 240provides the output of the video decoder 200. The output of the summingjunction 240 is connected in signal communication with a referencepicture store 250. The reference picture store 250 is connected insignal communication with a motion compensator 260, which is connectedin signal communication with a first input of a multiplier 270.

The VLD 210 is further connected in signal communication with areference picture weighting factor lookup 280 for providing an adaptivebi-prediction (“ABP”) coefficient index to the lookup 280. A firstoutput of the lookup 280 is for providing a weighting factor, and isconnected in signal communication to a second input of the multiplier270. The output of the multiplier 270 is connected in signalcommunication to a first input of a summing junction 290. A secondoutput of the lookup 280 is for providing an offset, and is connected insignal communication to a second input of the summing junction 290. Theoutput of the summing junction 290 is connected in signal communicationwith a second input terminal of the summing junction 240.

Turning now to FIG. 3, a video decoder with reference picture weightingis indicated generally by the reference numeral 300. The video decoder300 includes a VLD 310 connected in signal communication with an inversequantizer 320. The inverse quantizer 320 is connected in signalcommunication with an inverse transformer 330. The inverse transformer330 is connected in signal communication with a first input terminal ofa summing junction 340, where the output of the summing junction 340provides the output of the video decoder 300. The output of the summingjunction 340 is connected in signal communication with a referencepicture store 350. The reference picture store 350 is connected insignal communication with a motion compensator 360, which is connectedin signal communication with a first input of a multiplier 370.

The VLD 310 is further connected in signal communication with areference picture weighting factor lookup 380 for providing a referencepicture index to the lookup 380. A first output of the lookup 380 is forproviding a weighting factor, and is connected in signal communicationto a second input of the multiplier 370. The output of the multiplier370 is connected in signal communication to a first input of a summingjunction 390. A second output of the lookup 380 is for providing anoffset, and is connected in signal communication to a second input ofthe summing junction 390. The output of the summing junction 390 isconnected in signal communication with a second input terminal of thesumming junction 340.

As shown in FIG. 4, a standard video encoder is indicated generally bythe reference numeral 400. An input to the encoder 400 is connected insignal communication with a non-inverting input of a summing junction410. The output of the summing junction 410 is connected in signalcommunication with a block transformer 420. The transformer 420 isconnected in signal communication with a quantizer 430. The output ofthe quantizer 430 is connected in signal communication with a variablelength coder (“VLC”) 440, where the output of the VLC 440 is anexternally available output of the encoder 400.

The output of the quantizer 430 is further connected in signalcommunication with an inverse quantizer 450. The inverse quantizer 450is connected in signal communication with an inverse block transformer460, which, in turn, is connected in signal communication with areference picture store 470. A first output of the reference picturestore 470 is connected in signal communication with a first input of amotion estimator 480. The input to the encoder 400 is further connectedin signal communication with a second input of the motion estimator 480.The output of the motion estimator 480 is connected in signalcommunication with a first input of a motion compensator 490. A secondoutput of the reference picture store 470 is connected in signalcommunication with a second input of the motion compensator 490. Theoutput of the motion compensator 490 is connected in signalcommunication with an inverting input of the summing junction 410.

Turning to FIG. 5, a video encoder with reference picture weighting isindicated generally by the reference numeral 500. An input to theencoder 500 is connected in signal communication with a non-invertinginput of a summing junction 510. The output of the summing junction 510is connected in signal communication with a block transformer 520. Thetransformer 520 is connected in signal communication with a quantizer530. The output of the quantizer 530 is connected in signalcommunication with a VLC 540, where the output of the VLC 440 is anexternally available output of the encoder 500.

The output of the quantizer 530 is further connected in signalcommunication with an inverse quantizer 550. The inverse quantizer 550is connected in signal communication with an inverse block transformer560, which, in turn, is connected in signal communication with areference picture store 570. A first output of the reference picturestore 570 is connected in signal communication with a first input of areference picture weighting factor assignor 572. The input to theencoder 500 is further connected in signal communication with a secondinput of the reference picture weighting factor assignor 572. The outputof the reference picture weighting factor assignor 572, which isindicative of a weighting factor, is connected in signal communicationwith a first input of a motion estimator 580. A second output of thereference picture store 570 is connected in signal communication with asecond input of the motion estimator 580.

The input to the encoder 500 is further connected in signalcommunication with a third input of the motion estimator 580. The outputof the motion estimator 580, which is indicative of motion vectors, isconnected in signal communication with a first input of a motioncompensator 590. A third output of the reference picture store 570 isconnected in signal communication with a second input of the motioncompensator 590. The output of the motion compensator 590, which isindicative of a motion compensated reference picture, is connected insignal communication with a first input of a multiplier 592. The outputof the reference picture weighting factor assignor 572, which isindicative of a weighting factor, is connected in signal communicationwith a second input of the multiplier 592. The output of the multiplier592 is connected in signal communication with an inverting input of thesumming junction 510.

Turning now to FIG. 6, an exemplary process for decoding video signaldata for an image block is indicated generally by the reference numeral600. The process includes a start block 610 that passes control to aninput block 612. The input block 612 receives the image block compresseddata, and passes control to an input block 614. The input block 614receives at least one reference picture index with the data for theimage block, each reference picture index corresponding to a particularreference picture. The input block 614 passes control to a functionblock 616, which determines a weighting factor corresponding to each ofthe received reference picture indices, and passes control to anoptional function block 617. The optional function block 617 determinesan offset corresponding to each of the received reference pictureindices, and passes control to a function block 618. The function block618 retrieves a reference picture corresponding to each of the receivedreference picture indices, and passes control to a function block 620.The function block 620, in turn, motion compensates the retrievedreference picture, and passes control to a function block 622. Thefunction block 622 multiplies the motion compensated reference pictureby the corresponding weighting factor, and passes control to an optionalfunction block 623. The optional function block 623 adds the motioncompensated reference picture to the corresponding offset, and passescontrol to a function block 624. The function block 624, in turn, formsa weighted motion compensated reference picture, and passes control toan end block 626.

Turning now to FIG. 7, an exemplary process for encoding video signaldata for an image block is indicated generally by the reference numeral700. The process includes a start block 710 that passes control to aninput block 712. The input block 712 receives substantially uncompressedimage block data, and passes control to a function block 714. Thefunction block 714 assigns a weighting factor for the image blockcorresponding to a particular reference picture having a correspondingindex. The function block 714 passes control to an optional functionblock 715. The optional function block 715 assigns an offset for theimage block corresponding to a particular reference picture having acorresponding index. The optional function block 715 passes control to afunction block 716, which computes motion vectors corresponding to thedifference between the image block and the particular reference picture,and passes control to a function block 718. The function block 718motion compensates the particular reference picture in correspondencewith the motion vectors, and passes control to a function block 720. Thefunction block 720, in turn, multiplies the motion compensated referencepicture by the assigned weighting factor to form a weighted motioncompensated reference picture, and passes control to an optionalfunction block 721. The optional function block 721, in turn, adds themotion compensated reference picture to the assigned offset to form aweighted motion compensated reference picture, and passes control to afunction block 722. The function block 722 subtracts the weighted motioncompensated reference picture from the substantially uncompressed imageblock, and passes control to a function block 724. The function block724, in turn, encodes a signal with the difference between thesubstantially uncompressed image block and the weighted motioncompensated reference picture along with the corresponding index of theparticular reference picture, and passes control to an end block 726.

In the present exemplary embodiment, for each coded picture or slice, aweighting factor is associated with each allowable reference picturethat blocks of the current picture can be encoded with respect to. Wheneach individual block in the current picture is encoded or decoded, theweighting factor(s) and offset(s) that correspond to its referencepicture indices are applied to the reference prediction to form a weightpredictor. All blocks in the slice that are coded with respect to thesame reference picture apply the same weighting factor to the referencepicture prediction.

Whether or not to use adaptive weighting when coding a picture can beindicated in the picture parameter set or sequence parameter set, or inthe slice or picture header. For each slice or picture that usesadaptive weighting, a weighting factor may be transmitted for each ofthe allowable reference pictures that may be used for encoding thisslice or picture. The number of allowable reference pictures istransmitted in the slice header. For example, if three referencepictures can be used to encode the current slice, up to three weightingfactors are transmitted, and they are associated with the referencepicture with the same index.

If no weighting factors are transmitted, default weights are used. Inone embodiment of the current invention, default weights of (½, ½) areused when no weighting factors are transmitted. The weighting factorsmay be transmitted using either fixed or variable length codes.

Unlike typical systems, each weighting factor that is transmitted witheach slice, block or picture corresponds to a particular referencepicture index. Previously, any set of weighting factors transmitted witheach slice or picture were not associated with any particular referencepictures. Instead, an adaptive bi-prediction weighting index wastransmitted for each motion block or 8×8 region to select which of theweighting factors from the transmitted set was to be applied for thatparticular motion block or 8×8 region.

In the present embodiment, the weighting factor index for each motionblock or 8×8 region is not explicitly transmitted. Instead, theweighting factor that is associated with the transmitted referencepicture index is used. This dramatically reduces the amount of overheadin the transmitted bitstream to allow adaptive weighting of referencepictures.

This system and technique may be applied to either Predictive “P”pictures, which are encoded with a single predictor, or to Bi-predictive“B” pictures, which are encoded with two predictors. The decodingprocesses, which are present in both encoder and decoders, are describedbelow for the P and B picture cases. Alternatively, this technique mayalso be applied to coding systems using the concepts similar to I, B,and P pictures.

The same weighting factors can be used for single directional predictionin B pictures and for bi-directional prediction in B pictures. When asingle predictor is used for a macroblock, in P pictures or for singledirectional prediction in B pictures, a single reference picture indexis transmitted for the block. After the decoding process step of motioncompensation produces a predictor, the weighting factor is applied topredictor. The weighted predictor is then added to the coded residual,and clipping is performed on the sum, to form the decoded picture. Foruse for blocks in P pictures or for blocks in B pictures that use onlylist 0 prediction, the weighted predictor is formed as:Pred=W0*Pred0+D0  (1)

where W0 is the weighting factor associated with the list 0 referencepicture, D0 is the offset associated with the list 0 reference picture,and Pred0 is the motion-compensated prediction block from the list 0reference picture.

For use for blocks in B pictures which use only list 0 prediction, theweighted predictor is formed as:Pred=W1*Pred1+D1  (2)

where W1 is the weighting factor associated with the list 1 referencepicture, DO is the offset associated with the list 1 reference picture,and Pred1 is the motion-compensated prediction block from the list 1reference picture.

The weighted predictors may be clipped to guarantee that the resultingvalues will be within the allowable range of pixel values, typically 0to 255. The precision of the multiplication in the weighting formulasmay be limited to any pre-determined number of bits of resolution.

In the bi-predictive case, reference picture indexes are transmitted foreach of the two predictors. Motion compensation is performed to form thetwo predictors. Each predictor uses the weighting factor associated withits reference picture index to form two weighted predictors. The twoweighted predictors are then averaged together to form an averagedpredictor, which is then added to the coded residual.

For use for blocks in B pictures that use list 0 and list 1 predictions,the weighted predictor is formed as:Pred=(P0*Pred0+D0+P1*Pred1+D1)/2  (3)

Clipping may be applied to the weighted predictor or any of theintermediate values in the calculation of the weighted predictor toguarantee that the resulting values will be within the allowable rangeof pixel values, typically 0 to 255.

Thus, a weighting factor is applied to the reference picture predictionof a video compression encoder and decoder that uses multiple referencepictures. The weighting factor adapts for individual motion blockswithin a picture, based on the reference picture index that is used forthat motion block. Because the reference picture index is alreadytransmitted in the compressed video bitstream, the additional overheadto adapt the weighting factor on a motion block basis is dramaticallyreduced. All motion blocks that are coded with respect to the samereference picture apply the same weighting factor to the referencepicture prediction.

These and other features and advantages of the present invention may bereadily ascertained by one of ordinary skill in the pertinent art basedon the teachings herein. It is to be understood that the teachings ofthe present invention may be implemented in various forms of hardware,software, firmware, special purpose processors, or combinations thereof.

Most preferably, the teachings of the present invention are implementedas a combination of hardware and software. Moreover, the software ispreferably implemented as an application program tangibly embodied on aprogram storage unit. The application program may be uploaded to, andexecuted by, a machine comprising any suitable architecture. Preferably,the machine is implemented on a computer platform having hardware suchas one or more central processing units (“CPU”), a random access memory(“RAM”), and input/output (“I/O”) interfaces. The computer platform mayalso include an operating system and microinstruction code. The variousprocesses and functions described herein may be either part of themicroinstruction code or part of the application program, or anycombination thereof, which may be executed by a CPU. In addition,various other peripheral units may be connected to the computer platformsuch as an additional data storage unit and a printing unit.

It is to be further understood that, because some of the constituentsystem components and methods depicted in the accompanying drawings arepreferably implemented in software, the actual connections between thesystem components or the process function blocks may differ dependingupon the manner in which the present invention is programmed. Given theteachings herein, one of ordinary skill in the pertinent art will beable to contemplate these and similar implementations or configurationsof the present invention.

Although the illustrative embodiments have been described herein withreference to the accompanying drawings, it is to be understood that thepresent invention is not limited to those precise embodiments, and thatvarious changes and modifications may be effected therein by one ofordinary skill in the pertinent art without departing from the scope orspirit of the present invention. All such changes and modifications areintended to be included within the scope of the present invention as setforth in the appended claims.

1. A method for decoding video data for a picture having a plurality ofimage blocks, the video data including data for an image block of theplurality of image blocks and including a first reference picture indexand a second reference picture index, the method comprising: receivingthe video data, the received video data including the data for the imageblock, the first reference picture index for the image block, and thesecond reference picture index for the image block, the first referencepicture index corresponding to a first reference picture, and the secondreference picture index corresponding to a second reference picture; andpredicting the image block using the first and second referencepictures, and using a first offset corresponding to the first referencepicture index, and using a second offset corresponding to the secondreference picture index, wherein the first reference picture indexdetermines both the first reference picture and the first offset, andthe second reference picture index determines both the second referencepicture and the second offset.
 2. A method as defined in claim 1 whereinpredicting the image block comprises: motion compensating a block fromthe first reference picture; weighting the motion compensated block fromthe first reference picture using a first weighting factor; determiningthe first offset; motion compensating a block from the second referencepicture; weighting the motion compensated block from the secondreference picture using a second weighting factor; determining thesecond offset; and adjusting the weighted motion compensated block fromthe first reference picture and the weighted motion compensated blockfrom the second reference picture using the first offset and the secondoffset.
 3. A video decoder for decoding video data for a picture havinga plurality of image blocks, the video data including data for an imageblock of the plurality of image blocks and including a first referencepicture index and a second reference picture index, the decodercomprising: means for receiving the video data, the received video dataincluding the data for the image block, the first reference pictureindex for the image block, and the second reference picture index forthe image block, the first reference picture index corresponding to afirst reference picture, and the second reference picture indexcorresponding to a second reference picture; and means for predictingthe image block using the first and second reference pictures, and usinga first offset corresponding to the first reference picture index, andusing a second offset corresponding to the second reference pictureindex, wherein the first reference picture index determines both thefirst reference picture and the first offset, and the second referencepicture index determines both the second reference picture and thesecond offset.
 4. A computer readable media readable by machine,tangibly embodying a program of instructions executable by the machineto perform program steps for decoding video data for a picture having aplurality of image blocks, the video data including data for an imageblock of the plurality of image blocks and including a first referencepicture index and a second reference picture index, the program stepsincluding: receiving the video data, the received video data includingthe data for the image block, the first reference picture index for theimage block, and the second reference picture index for the image block,the first reference picture index corresponding to a first referencepicture, and the second reference picture index corresponding to asecond reference picture; and predicting the image block using the firstand second reference pictures, and using a first offset corresponding tothe first reference picture index, and using a second offsetcorresponding to the second reference picture index, wherein the firstreference picture index determines both the first reference picture andthe first offset, and the second reference picture index determines boththe second reference picture and the second offset.